1. Field of the Invention
The present invention relates to video signal recording/reproducing apparatus wherein visually satisfactory images can be obtained without generating noise bands and reducing the signal-to-noise ratio in case of a high or changed speed reproduction mode, such as the search mode.
2. Description of the Prior Art
Enhancement in quality of a picture based on changed or high speed reproduction of the VTR taking a recording system (one-channel/one-segment recording system) for recording video signals for one field on one track of a magnetic tape has heretofore been attained by highly accurate control of the travelling of the magnetic tape or by use of special reproduction heads. With recent tendencies to a decrease in price of semiconductor products such as memories and to higher speed/higher density thereof, some VTRs have been offered incorporating a field memory to execute changed speed reproduction. The changed speed reproduction making use of the field memory will be explained as follows.
FIG. 1 is a schematic diagram illustrating a conventional changed speed reproduction system of a VTR which employs the field memory. Information which has been recorded on a magnetic tape 1 is reproduced by means of rotary heads 1a and 1b having different azimuth angles to each other. Outputs of the rotary heads 1a and 1b are transmitted to and amplified by head amplifiers 2a and 2b. Any one of the outputs of the head amplifiers 2a and 2b is selected by a switching circuit 3. The output selected by the switching circuit 3 is given to a video signal processing circuit 4 which FM-demodulates the output from switching circuit 3. Then a video signal is reproduced and at the same time a synchronization signal is detected.
The output selected by the switching circuit 3 is also transmitted to an envelope detecting circuit 5, thereby obtaining an envelope of the reproduced FM signal. The outputs of the envelope detecting circuit 5 are supplied to a comparator 6 to be compared with a predetermined level. A resulting signal from the comparator 6 is sent to a memory control circuit 7. Then, the memory control circuit 7 supplies a write-enable signal to a field memory 8 to carry out write-control on the basis of the synchronization signal emitted from the video signal processing circuit 4 and of the control signal from the comparator 6, and further performs readout-control in asynchronism with the synchronization signal.
A control head 9 is provided and serves to read a second control signal recorded on the magnetic tape 1 in order to transmit it to a travelling control circuit 10. The travelling control circuit 10 sends a drive signal to a motor driver 11 on the basis of the second control signal read by the control head 9. Motor driver 11 drives a capstan motor 12. The rotation frequency of this capstan motor 12 is detected and sent to the travelling control circuit 10.
Next, the operation of the reproduction system of the prior art VTR will be described with reference to FIGS. 1 to 3 and in case of the high speed reproduction which is effected at an N-fold speed [N=.+-.2, .+-.4, .+-.6, . . . ; (.+-.) indicates the travelling direction, (+) indicates that reproduction is made in the same direction as that in the ordinary reproduction, and (-) shows that the direction of reproduction is opposite to the ordinary direction]. FIG. 2(a) shows the relation between the recording tracks and the scanning loci of the heads when N equals six and the magnetic tape runs in the direction indicated by the arrow X.
In FIGS. 2(a) and 2(b), "R" and "L" written on the tracks indicate that the portions marked with "R" and "L" have been recorded by the rotary heads Ia and 1b or by other heads having the same azimuth angle as rotary heads 1a and 1b. The signals recorded on the portions depicted with oblique lines are reproduced by the respective heads owing to the azimuth effect. The comparator 6 compares the outputs from the envelope detecting circuit 5 with the predetermined level and then outputs a signal to the memory control circuit 7 when the former exceeds the latter. Thus the memory control circuit 7 operates to write the outputs of the video signal processing circuit 4 to the field memory 8 when the output of the envelope detecting circuit 5 exceeds the predetermined level. It follows therefore that in the field memory 8 the video signals corresponding to one field are recorded in a pattern such as illustrated in FIG. 2(b).
On the other hand, the high definition television system which has been developed as a television system for the next generation is now being put into practical use. Such a high definition television system has a wide video signal band of approximately 20 MHz. As a means for recording such a wide band signal, the multichannel/multisegment recording system has been employed. Namely, the formation into multichannels contributes to reduction in the signal band width of one channel, while the formation into multisegments contributes to a rise in the speed of the magnetic tape relative to that of the rotary head, with the result that the recorded wavelength is increased. With this arrangement, the wide band signals are recorded and reproduced by the conventional magnetic recording technology.
An example wherein the two-channel/three-segment recording system is employed will be explained hereinafter.
FIGS. 3(a) to 3(d) show the concept of the two-channel/three-segment recording system. In FIG. 3(a) illustrating the disposition of the rotary heads, the numeral 13 denotes a rotary drum; 14a and 14b represent rotary heads for recording/reproducing the signals of channel A (hereinafter referred to as CH. A); 15a and 15b designate the rotary heads for recording/reproducing the signals of channel B (hereinafter referred to as CH. B), the rotary heads 14a, 15a and 14b, 15b being adjacently disposed. During recording, the rotary heads simultaneously perform the recording operation on both the channels A and B, as illustrated in FIG. 3(d). The magnetic tape is wound over the range of 180.degree. or more on the rotary drum 13 in recording and reproducing. The rotary drum 13 makes one and a half revolutions in the direction indicated by an arrowhead A in FIG. 3a during the one-field period, whereby the video signals of CHs. A and B for one field are recorded in the form of the split three segments.
As shown in FIG. 3(c), an input video signal such as the composite signal of one channel is allocated sequentially from above to the individual segments of the respective channels by employing the field memory on the basis of the unit of one horizontal scanning period. Therefore, a picture is, as illustrated in FIG. 3(b), divided into the total of six pictures, and each of the pictures thus obtained is recorded on one track of the magnetic tape.
The reference symbols A1, A2, . . . recorded on the tracks shown in FIG. 3(d) designate pieces of image information of the first, the second, . . . segments . . . of CH. A.
It is to be noted that it is possible to attain in a relatively easy manner high speed reproduction in a VTR which adopts the multichannel/multisegment recording system where the recording is effected according to the above described tape format. To be specific, the lower part of the track corresponds to the upper part of the picture, the central part corresponds to the central part of the picture, and the upper part of the track corresponds to the lower part of the picture. For this reason, even if the order in which the six pictures shown in FIG. 3(b) are superimposed on each other is slightly changed, no visual problem arises because in the high speed reproduction the motion of images appears to be unnatural. Namely, where the fetch into the field memory is performed, when reproducing, for instance, the information A1, the reproduced outputs may not be input to the exact address into which the information A1 should be written. Instead, the image information obtained by reproducing, for example, the information A3, is input with respect to the address with which the information A1 should be input. This does not cause serious visual problems.
In the changed speed reproduction from the VTR which takes the multichannel/multisegment recording system having the above-described recording format, if the full video information on the track is completely reproduced in a cycle within a given period, it is possible to actualize satisfactory reproduction with no noise in the changed speed reproduction, even if there is some unnaturalness.
FIG. 4 is a block diagram of a prior art video signal recording/reproducing apparatus which adopts the two-channel/three-segment recording system. In the figure, the same reference numerals as used in FIG. 1 denote like elements. Reference numerals 16a and 16b designate rotary heads for the CHs. A and B; reference numerals 17a and 17b designate rotary heads for the CHs; and 4a and 4b stand for video signal processing circuits which produce a video signal by effecting FM-demodulation and detect a synchronous signal for A/D conversion. The reference numerals 7a and 7b stand for memory control circuits for executing write-control in response to the synchronization signal emitted from the video signal processing circuit 4a and 4b and for executing read-control in asynchronism with the synchronization signal.
Field memory 8 comprises two memories 8a and 8b for CHs. A and B, respectively. The reference numeral 18 stands for a selector.
Next, the operation of the apparatus of FIG. 4 will be explained. Where the high speed reproduction is carried out at a speed N times the recording speed (N=.+-.2, .+-.3, . . . : (.+-.) indicates the travelling direction. (+) shows the same direction as that of the ordinary reproduction, and (-) shows the opposite direction), and when N=4, the relation between recording tracks and the head scanning loci is established as illustrated in FIG. 5(a). If the rotary heads for CHs. A and B have different azimuth angles, the signals which are to be reproduced by the respective heads by virtue of azimuth effect comprise those obtained from the portions depicted with the oblique lines on the tracks. Turning our attention to FIG. 5(b), reproduction envelopes to be emitted are shown. In the figure, no reproduction output is obtained in the portions indicated by the oblique lines. As explained earlier, the video signal is recorded in such a tape format that the lower part of the track corresponds to the upper part of the picture, the central part corresponds to the central part of the picture, and the upper part corresponds to the lower part. Hence, the video information in the field memory cannot be rewritten with respect to the portions depicted with the oblique lines in FIG. 5(b). As a result, the fixed noise bands n, or the noise bars shown in FIG. 5(c) are produced on the picture p.
Once such fixed noise bands n appear, the problem of being unable to obtain visually favourable images arises.
An example of a two-channel/three-segment recording system is given below. The input image signals (composite signals of one channel) are allocated to two channels (CH. A and Ch. B) on the basis of a unit of one horizontal scanning period (hereinafter referred to as 1H) and the length of the time axis is doubled. As a result, the signal band width of each channel is reduced to half the original width.
FIG. 6(a) shows the placement of the rotary heads. In FIG. 6(a), a rotary drum 13 is provided with rotary heads 14a, and 14b for recording/reproducing the signals of CH. A and rotary heads 15a and 15b for recording/reproducing the signals of CH. B. The two pairs of rotary heads 14a15a and 15b are contiguously disposed. The magnetic tape is wound at an angle of 180.degree. on the rotary drum 13 at the recording/reproducing time. The rotary drum 13 makes one and a half revolutions during one field period. Hence, it follows that the image signals for one field are recorded so that the signals of CHs. A and B are respectively split into three sections (three segments).
FIG. 6(b) shows a concept of the two-channel/three-segment recording system. FIG. 6(c) illustrates a recording track pattern formed on the magnetic tape. A1, A2 and A3 shown on the tracks in FIGS. 6(b) and 6(c) indicate the image signals of the first, second and third segments which are recorded by mans of the rotary heads 14a and 14b corresponding to channel A.
The high speed reproduction in which the tape traveling velocity is, as in the prior art, set to be an integer multiple is now considered. When N=2, the relation between the recording tracks and the scanning loci of the heads is as shown FIG. 6(d). Where the rotary heads 14a, 14b, 15a and 15b have the same azimuth, no azimuth effect is present. Hence, it is possible to reproduce the image signals of all the tracks that the heads have traversed. While, on the other hand, as can be seen in the potions depicted with the oblique lines in FIG. 6(e), there are sections in which the signals recorded by the rotary heads 14a and 14b and the signals recorded by the rotary heads 15a and 15b are simultaneously read. The image signals are superposed on each other in these sections, whereby favorable outputs cannot be obtained.
Considering such circumstances, the output envelopes of the first segment CH. A are shown in FIG. 6(f). In the figure, the superposition takes place in the portions indicated by the oblique lines, and hence no good picture is obtained.
FIG. 6(g) illustrates and actual picture. The portions shown by the oblique lines present a situation where the image information of the field memory is not rewritten, and fixed noise bands are created such that a visually good picture is not acquired.